Today, fiberglass products usually are produced using a chopper type system in which a polyester or similar epoxy resin, the appropriate catalyst and glass fiber stock are fed to a chopper gun from which these components are projected onto a mold. In cases where product weight is not critical, as, for example, bath tubs and shower stalls, the industry recognizes the desirability of incorporating in the resin a filler, such as hydrated alumina, antimony trioxide, glass beads, talc minerals or various carbonates and silicates. These fillers impart fire retardant properties to the product and also reduce considerably the amount of the relatively expensive resin which is needed. However, in the past, incorporation of fillers has presented certain difficulties, several of which are attributable directly to the fact that the filled resin has a relatively high viscosity. For example, whereas a typical unfilled polyester resin may have a viscosity of about 100 centipoises, the filled version of this resin, containing 50% by weight of hydrated alumina, may exhibit a viscosity on the order of 1,600 centipoises. This great difference makes necessary some modifications of the chopper system, namely, the removal of some filter screens from the gun, and the substitution of a larger resin hose and jet. Furthermore, the increase in viscosity necessitates a reduction of the glass content of the product from the normal 30% by weight to a level of about 20%. This, of course, decreases the tensile strength of the product. Another problem encountered with filled resins is the formation and build-up of solid, rubber-like deposits in the mixing apparatus, which necessitates periodic shut-down of the equipment for cleaning purposes. Finally, prior systems using filled resin do not allow this component to be pumped directly from the mixer to the chopper gun and are characterized by such rapid settling of the filler that around-the-clock operation of the mixer is a necessity.
The object of this invention is to provide an improved technique for using filled resins in chopper type fiberglass systems. The invention recognizes the fact that the rather vigorous agitating action of prior resin-filler mixers results in excessive evaporation of the volatile styrene constituent of the resin, and thus causes the marked increase in viscosity noted above. Accordingly, the new technique employs a special mixer which is capable of affording thorough and rapid mixing of the resin and filler in a gentle fashion. The new mixing apparatus comprises a vertical tank having a cylindrical side wall and a flat bottom, and which contains a coaxial rotor equipped with at least two axially spaced sets of equiangularly spaced, radial blades. One set of blades is positioned adjacent the bottom of the tank, and the other is located at an intermediate elevation. Each rotor blade is inclined over its entire length at an angle on the order of 45.degree. with respect to the horizontal and in a direction which causes it to lift the mixture in the tank. The tank is equipped with a set of imperforate, radial baffles which are attached to the cylindrical wall and are spaced equiangularly about its axis. These baffles extend vertically from a region adjacent the bottom of the tank to an elevation slightly below the top of the tank, and they extend inward to a region closely adjacent the tips of the rotor blades. A prime mover carried by the tank drives the rotor at a speed which produces a relatively low tangential velocity on the order of 7 to 9 feet per second at the blade tips. Preferably, the mixing apparatus is equipped with a vertical discharge pipe which extends into the tank from above and is located at the downstream side of one of the baffles in the corner defined by it and the cylindrical tank wall.
Actual use of the new technique has revealed the following advantages:
1. Thorough mixing of the resin and filler is accomplished rapidly, e.g., in about 20 minutes. PA1 2. The mixture, even in the case where 50% by weight of filler is employed, has a viscosity which is about one-half that of prior filled resins, and which permits use of the conventional chopper equipment employed with unfilled resins. PA1 3. In the case of the 50--50 resin-filler mixture, a glass content of about 27% by weight in the product may be used. PA1 4. There is no appreciable build-up of rubber-like deposits in the mixing tank. PA1 5. The settling time of the mixture is considerably longer than in the past, being on the order of 2 - 7 hours depending upon the brand of resin employed. Moreover, the mixture could be stored in an idle tank for a period as long as 10 days and then brought to a useable state by merely operating the rotor for the usual 20-minute mixing cycle. PA1 6. The equipment is relatively inexpensive and is sufficiently compact that a separate mixer may be installed at each spray booth. Furthermore, the filled resin could be pumped directly from the tank to the chopper gun, so an auxiliary holding vessel was not needed.